The invention generally relates to a mill intended to be driven by a flow of fluid. More in particular the invention relates to a windmill of the so-called horizontal or Chinese type. Windmills of this type comprise a vane rotor mounted for rotation in a stationary frame about a generally vertically extending central axis of this frame. The vane rotor supports a plurality of vane shafts rotatably mounted in the rotor and arranged in an annular array about the rotor axis, generally extending parallel to this axis. Each vane shaft carries a vane member with at least a major portion of the vane extending to one side of the shaft. Each vane may for instance comprise a rigid rectangular frame with a flexible sheet secured therein which frame is attached to its vane shaft along or adjacent one side member of the frame. The vane rotor further comprises stop means for each vane determining an operating position of the vane when held by the stop means while apart from this stop means the vane is free to swing about its vane shaft. In the operation of such a windmill the wind blowing from any direction but perpendicular to the axis of the vane rotor swings the vanes on one side of the rotor axis to their operating positions in which they engage the stop means and thus are held at an angle to the wind, generally in substantially radial planes whereas the vanes at the opposite side of the rotor axis are free to swing in positions parallel to the direction of the wind. The vanes in the operating position thus move with the wind and drive the vane rotor and the vanes in the free inoperative position move against the wind with a minimum of resistance. These latter vanes when reaching the weather side of the mill are gradually rotated about their vane shafts until they engage their stop means in the operating position while the vanes reaching the lee-side of the mill are swung around with a gybing movement from their operating position to a position parallel to the wind. The stop means can be so arranged that the vanes in their operating position extend substantially radially inwardly from their vane shafts towards the rotor axis in which case the vanes provide a driving couple through an angle of about 180.degree. of their rotational movement about the rotor axis. In another embodiment of the Chinese mill, however, the stop means consist of a string attached to the vanes so as to restrict their rotational movement about the vane shaft to an angle of about 150.degree. in such a way that in the operating position the vanes extend at a small angle to a radial line through the rotor axis and the vane shaft and that after gybing the vanes are at first held by the string in a position at an angle to the direction of the wind in which the vanes still contribute to the driving couple. In this arrangement the vanes each contribute a driving couple over an angular part greater than 180.degree. of each revolution about the rotor axis.
In both cases, this type of windmill has the disadvantage that the vanes when passing the lee-side of the mill make an uncontrolled gybing movement which may cause shocks and vibrations and an uneven running of the vane rotor, particularly in strong winds. This is one of the reasons why the known windmills of the horizontal type are not suitable for running at high speeds as are required, for instance, for driving an electric generator. Furthermore, the known windmills of the horizontal type have difficulty in starting at a light and changing wind since generally it will take a full revolution of the vane rotor before all of the vanes have swung into the correct position for the wind direction concerned and before that time the vanes can only produce a restricted driving couple.